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Virtual appliances are great for the same reasons physical appliances took the IT world by storm: They make deployment a snap -- even instantaneous -- while at the same time reducing costs. It's a formula that made hardware-based appliances immensely popular for network security, backup, storage networking, file services, email, and many other single-focus solutions.

By leveraging open source solutions, and shedding the hardware entirely, virtual appliances can push the cost savings to whole new level: completely free. You'll find an increasing number of open source projects available for download in virtual machine formats for Xen, VirtualBox, VMware, and other virtualization platforms. Naturally, the virtual appliance is especially handy when the solution stitches together a host of components or relies on an entire LAMP stack. Why build it yourself when you can download the whole installation, already pre-configured?

In short, with the rise in popularity of virtualization and server consolidation, IT pros have many other options, options that offer all of the pros of a physical appliance, with very few of the drawbacks. Even when the software is proprietary, a virtual appliance costs less than a physical hardware appliance. And then there are all the other advantages of virtualization: The ability to run multiple virtual appliances on a single server, the ability to migrate virtual appliances from one server to another, and the ability to back up a virtual appliance almost instantly. All of these capabilities play well into disaster recovery and business continuity planning and help make virtual appliances a "must consider" technology for businesses large and small.

Before diving into the widening world of virtual appliances, there is no better way to experiment than with virtual appliances that are free to use and meet unique needs in an enterprise. Listed below are some of the free standouts that we have come across over the years, all of which offer enough features and capabilities to make the transition from an experimental virtual appliance to a line-of-business solution.

Openfiler NAS and SAN Openfiler is a full-fledged NAS/SAN appliance (or IP storage gateway) that can be downloaded as a virtual appliance for VMware and Xen hypervisors directly from openfiler.com. Openfiler offers storage administrators a set of powerful tools that are used to manage complex storage environments. The product brings reliability to storage with support for both software and hardware RAID, monitoring and alerting facilities, and volume snapshot and recovery features. High availability is included in the form of active/passive clustering, multipath I/O, and block level replication. Scalability is also addressed by a filesystem that can support up to 60TB, with the ability to expand filesystems and volumes without taking storage offline. High performance is guaranteed by a Linux 2.6 kernel that supports the latest CPU, networking, and storage hardware.

Configuring Openfiler can be rather complex, but there are plenty of resources available on the Web that cover the most typical installations. Openfiler is managed using a browser-based console, which offers dashboard-like simplicity and several submenus to address the more complex configuration settings available. When paired with VMware ESXi, Openfiler brings enterprise level storage capabilities -- including iSCSI and other SAN and NAS services -- to most any network, completely free of charge. Support plans and commercial add-ons are available at www.openfiler.com for those looking to add additional capabilities or receive professional technical support.

Opsview monitoring Opsview Community Edition 3.8 is free, open source enterprise network and application monitoring software that is available as a virtual appliance for VMware player. However, it can be converted to run on VMware ESX or ESXi by using the free VMware vCenter Converter tool.

Opsview is built on Nagios, an open source enterprise management platform. Opsview adds enhancements to notifications and alerting (both in terms of controls and methods), support for SNMPv3 with MRTG, flap detection, faster importing, and improved uptime checks. In addition to the VMware appliance, Opsview Community Edition can be downloaded as source or binaries for RHEL 5, CentOS 5, Debian, Ubuntu, and Solaris 10.

Opsview offers some unique features that make it a formidable management tool, such as support for monitoring virtual servers and a variety of hardware components found throughout the network. Normally, an advanced management tool like Opsview would be difficult to install and configure -- however, most of the pain of installation is eliminated with the virtual appliance version of the product, where initial setup takes only a matter of minutes. The browser-based GUI is informative and easy to navigate, making it a snap to add devices to be monitored and to define dashboards that offer meaningful information. The Community edition is offered under a GPL license, while a more advanced version of the product that includes support is offered under a subscription model. Managers of smaller, simpler networks may find everything in the free version to meet their needs.

Cacti network graphing Cacti is a network monitoring and performance graphing tool that works with SNMP to capture what is happening on the network and present that information in easy-to-understand graphs. As with any network monitoring tool, Cacti can be somewhat complex to set up, because of the dependencies on MySQL, PHP, and other components. However, for those administrators who want a visual representation of what is happening on the network performance wise, it is pretty hard to beat the flashy, informative charts that Cacti can offer. &nbsp;

Cacti works by polling all SNMP-capable devices on your network and adding the information you've selected to graphs. In its simplest form, Cacti will give you the ability to create graphs for the most common aspects of host monitoring, such as disk space, loads, memory usage, NIC monitoring (bytes in/out of an interface), and so on. The browser based management console is nicely done, and plenty of support is offered on www.cacti.net.

Downloading Cacti as a virtual appliance avoids a somewhat complex configuration process that includes creating a virtual server, installing CentOS, and configuring Cacti. However, preconfigured virtual appliances usually cost a few dollars, as opposed to being free. An example of that would be the JumpBox version of Cacti, which runs $49 or $99 per month; a 7 day trial is available at no charge if you want to give that implementation a try, before going through the manual steps to create your own appliance.

M0n0wall firewallM0n0wall is an all-in-one network firewall package that includes many of the features found in commercial firewalls, including stateful packet filtering, NAT/PAT, DHCP, PPTP VPN, SNMP, and IPSec. Originally intended to be used as an embedded software firewall on inexpensive devices, M0n0wall is both very compact, requiring only 12MB of disk space, and fast. That initial design goal also makes M0n0wall a good candidate for running as a virtual appliance.

M0n0wall is surprisingly full-featured and configurable, especially when you consider the price (free) and the small footprint. M0n0wall easily provides enough security controls and fine tuning abilities to make it suitable for branch offices and other small networks.

You can download the M0n0wall VM directly from the M0n0wall project site and instantly move it over to a virtual server. Setup and defining rules is relatively easy, thanks to a browser-based GUI that lays out all of the menus and choices in a clear and concise fashion. Anyone familiar with firewalls should have little trouble with M0n0wall.

AS Communication Gateway AS Communication Gateway offers anti-spam and email anti-virus protection for networks of any size. The product was developed in Europe and is available as a virtual appliance from the underground8 website. AS Communication Gateway provides a complete email security solution, removing known and unknown threats like spam, phishing, worms, and Trojans without major administration and user effort. Because it sits between the mail server and incoming email traffic, it filters out the bad stuff even before delivery to the server.

The AS Communication Gateway uses the ASMX engine for spam and malware detection. The product features 20 filters for spam detection and reliably blocks as much as 80 percent of incoming spam. Features include an adaptive email quarantine for each users, where the user can see the quarantine contents and spam digests displayed using an intuitive interface. The quarantine operates automatically and sets up the correct protection mechanisms for each user based on what the user does and does not flag as spam.

A botnet filter is included as well as dual layer anti-virus scanning. AS Communication Gateway also includes protection against DoS attacks and -- for a price -- can integrate the anti-virus engine from Kaspersky Labs.

Normally, a CMS is complex to install, configure, and use, but WebGUI overcomes those three challenges by offering the server as a virtual appliance that runs under VMware's virtualization products. A community-based, open source approach has steadily improved the product to the point where its browser based interface is one of the easiest for end users to use. That same interface powers administrative features, keeping setup and changes relatively easy.

WebGUI includes a multitude of integrated applications that allow users to build websites and intranets the way they want. There are tools for creating e-commerce sites, sites with dynamic content, and editors for end users that eliminate the need to learn HTML to create Web content. WebGUI is one of the most complete CMS products available, and will require little modification to meet an organization's basic Web publishing needs. It will prove to be ideal for nonprofits and other businesses with limited budgets.

BitNami WordPress WordPress is a Web publishing platform that focuses on aesthetics, Web standards, and usability, which is a long way of saying WordPress makes it easy to put content up on the Web. Although WordPress has been around since 2003, it has been rather complex to set up and deploy. However, BitNami offers WordPress (and many other applications) as a VMware virtual appliance, which contains a minimal Linux operating system (either OpenSuse or Ubuntu) and the fully configured Apache-MySQL-PHP stack that WordPress requires.&nbsp;

WordPress specializes in creating "static" Web content, i.e. Web pages with a focus on text. Thousands of themes and plug-ins are available to help users create custom sites that organize information in the way they prefer. WordPress offers a browser based dashboard that includes tools to edit and post content, manage images, and track comments. Most users will find WordPress easy to use and a simple way to manage their own content online.

As a virtual appliance, WordPress can be deployed in a matter of minutes, and users can be creating content for the Web or an intranet within an hour. WordPress is completely free and several distributions of it are available, allowing users to transition from the virtual appliance to a full fledged Linux implementation of the product. However, most smaller, straightforward sites may never need to move away from BitNami's virtual appliance version.

FOG computer cloningLinux-based FOG is free and open source computer imaging solution for Windows XP, Windows Vista, and Windows 7. It ties together a few open source tools with a PHP-based Web interface. FOG runs from a server and doesn't require any boot disks or CDs -- everything is done via TFTP and PXE. With FOG, most of the hardware drivers are built into the kernel, eliminating the need to manually install additional drivers to perform the imaging process on a PC. FOG offers some advanced features that are normally found only in commercial products, such as the ability to reduce partition sizes, resize drive images, and rename target computers.